The reason the clearances get tighter over time is because the valve and valve seat wear. The valve seats can actually get compressed into the head a tiny amount as well. What that does is cause the valve to sit higher in the head (the spring is holding it up) but the rocker arms are still in the same place from the last adjustment (or when it was built).

I always shoot for the tight side of whatever valve clearance specs you're given to try to keep things quiet. ...and if you think adjusting these valves are a pain in the ass, try adjusting the valves on a Yamaha R1. 5 valves per cylinder (total of 20 valves) and they're shim under bucket. No locknuts and adjusting screws. Gotta pull the cams and change the shims to adjust. Ugh.